Plasmids containing cloned herpesvirus thymidine kinase genes will be studied. Plasmids containing fragments of vaccinia DNA will be tested for transforming activity in LM (TK-) cells in order to identify the vaccinia DNA fragment that codes for TK. Genetic and biochemical studies will be carried out on plasmids with a 2kbp Pvu II fragment that codes for herpes simplex virus type 1. In cells transformed by this plasmid, and deletion mutants derived from it, the mRNA for TK will be isolated and the 5' terminal sequences will be identified. Attempts will be made to learn whether the thymidine kinase gene is transcribed from HSV or from flanking cellular DNA sequences.